Crystal oscillator with reduced acceleration sensitivity

ABSTRACT

A crystal oscillator having a plurality of quartz crystals that are manufactured so that the directional orientation of the acceleration sensitivity vector is essentially the same for each crystal. This enables convenient mounting of the crystals to a circuit assembly with consistent alignment of the acceleration vectors. The crystals are aligned with the acceleration vectors in an essentially anti-parallel relationship and can be coupled to the oscillator circuit in either a series or parallel arrangement. Mounting the crystals in this manner substantially cancels the acceleration sensitivity of the composite resonator and oscillator, rendering it less sensitive to vibrational forces and shock events.

RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/112,634, filed Nov. 7, 2008,entitled “CRYSTAL OSCILLATOR WITH REDUCED ACCELERATION SENSITIVITY”, theentirety of which is hereby incorporated by reference herein and made apart of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate generally to crystal oscillators andmore specifically to crystal oscillators with reduced accelerationsensitivity.

2. Description of the Related Art

Quartz crystals are commonly used to control the frequency of electronicoscillators. Although quartz is one of the most stable materialsavailable for fabricating a high frequency resonator, certainlimitations can become apparent in precision applications. For instance,changes in the ambient temperature cause the resonant frequency tochange. In addition, the natural frequency of a quartz resonator canalso be affected by applied acceleration forces. In some situations,these effects on the frequency are relatively small and can goundetected (˜0.0000002% per g of applied force). However, in manyapplications, an oscillator must operate in an environment whichsubjects it to levels of vibration or shock, where the resultantfrequency shifts can be significant and can limit system performance.These deleterious effects are a well known problem and a major concernof oscillator designers.

Acceleration forces applied to a crystal oscillator assembly will causea shift in the operating frequency. If these forces are in the form ofperiodic or sinusoidal vibration, frequency modulation will appear assidebands to the carrier at the vibration frequency, the amplitude ofwhich is determined by the amount of frequency shift. When theacceleration forces are in the form of random vibration, an increase inthe broadband noise floor of the oscillator will occur. Either of theseconditions may result in serious degradation of system performance in anoise sensitive application. Shock pulses due to handling or otherenvironmental events can cause a jump in the frequency which may resultin circuit malfunction such as loss of lock in phase locked loop or GPStracking applications.

There are generally two classes of methods to minimize the effects ofacceleration forces on crystal resonators. The first class is known asactive compensation. In active compensation, an acceleration sensor isused to detect the characteristics of applied forces and a signal isthen processed and fed back to the oscillator circuit to adjust thefrequency by an equal magnitude but in the opposite direction from theacceleration induced shifts. This method can be effective over certainvibration frequency ranges, but it requires a relatively complex circuitand can be very expensive to implement.

The second class is referred to as passive compensation. Passive methodsdo not attempt to sense the applied acceleration. Generally, in passivemethods, the crystal resonator or resonators are constructed usingspecial methods that render them somewhat insensitive to accelerationforces. Passive methods can be effective, but they generally require aninvolved and difficult fabrication process to produce the requiredcrystal or composite crystal assembly.

In view of these complications, one attempt has been to cancelacceleration sensitivity including determining a dominant sensitivityaxis of the resonators and then mounting the resonators with thedominant sensitivity axes in an anti-parallel arrangement. However, inaligning the resonators according to a supposedly dominant axis, suchmethods do not take into account the sensitivities along the other axes.As a result, the exact maximum magnitude and direction of a crystal'sacceleration sensitivity characteristic is not accounted for in suchmethods, and it is less effective in minimizing the effects ofacceleration forces.

SUMMARY OF THE INVENTION

It is more effective to account for the magnitude and direction of thetotal acceleration sensitivity vector by summing or taking into accountsensitivity in all three axes of the resonator. The accelerationsensitivity of a quartz crystal can be characterized as a vectorquantity, commonly denoted as Γ. The frequency shifts that are inducedby external acceleration are therefore determined by both the magnitudeand direction of the applied forces. The fractional frequency shift Δf/funder an acceleration {right arrow over (a)} is given by

$\frac{\Delta \; f}{f} = {\overset{\rightarrow}{\Gamma} \cdot {\overset{\rightarrow}{a}.}}$

By measuring the components of Γ in three mutually perpendiculardirections which are perpendicular to the faces of the crystal orresonator package, it is possible to calculate the exact maximummagnitude and direction of the Γ vector. Forces will have the mosteffect when they are imparted to the crystal in a direction that isparallel to this vector.

When two essentially identical crystals are aligned so that theirvectors are in opposing directions or anti-parallel and coupledelectrically in combination to the oscillator circuit, the vectors willcancel, rendering the composite resonator less sensitive to accelerationforces. Such an approach, however, has been difficult to achieve. Twocrystals must be carefully matched and physically oriented so that thevectors are anti-parallel. Crystal resonators exhibit substantialvariation in the direction and magnitude of the vector. The vectordirection can vary as much as 60° even with resonators that have beenidentically manufactured. Measurements of many crystals have shown thatthe acceleration vector is not oriented relative to the crystallographicaxes in any consistent manner even in identically designed andmanufactured crystals. Based on these challenges, it has been necessaryto physically manipulate the mounting orientation of the crystals toachieve the anti-parallel relationship. Vector inconsistency generallyrequires the use of complicated equipment such as an adjustable 3-axisgimbal mounting apparatus to individually align each crystal preciselyas needed to achieve significant cancellation. Therefore, achievingvector cancellation involves a difficult and time consuming process ofmeasuring, adjusting and mounting the crystals in an effective manner.Manufacturers have generally avoided this approach because it is sodifficult and expensive. Embodiments of the present invention eliminatethis burdensome process by configuring an oscillator with crystals thathave been manufactured so that the direction of the Γ vector points in aconsistent and predictable direction in each crystal, the directionbeing relative to the normal mounting plane.

Embodiments of the present crystal oscillator include a plurality ofcrystals mounted with the acceleration sensitivity vectors in anessentially anti-parallel relationship. This helps to cancel the effectsof acceleration or vibration on the output signal of the oscillator. Dueto the vector nature of this characteristic, when the crystals are soaligned, cancellation of the acceleration effects will occur.Embodiments of this oscillator use crystals which have been manufacturedso that the acceleration sensitivity vector points in a consistent andpredictable direction relative to the mounting surface of the resonator.

In one embodiment, crystals are selected which have the sameacceleration sensitivity vector magnitudes, within a certain tolerance.Complete cancellation of the sensitivity vectors can occur if thesensitivity vectors are of the exact same magnitude. However,significant cancellation can be achieved if the magnitudes of thesensitivity vectors differ.

The crystals are preferably contained in individual crystal packages orresonators. The crystals or resonators are then mounted to an oscillatorcircuit which is configured to sustain periodic oscillations. In oneembodiment, the crystals are coupled in pairs so that the first crystalis inverted with respect to the second crystal. This inversion can beachieved by rotating the first crystal 180° around either the x axis inthe y-z plane or around the y axis in the x-z plane. Because thecrystals have been manufactured so that the direction of the vector issubstantially the same for all crystals, the vectors can be aligned inan essentially anti-parallel manner without the need to measure andcharacterize the vector direction of each crystal and then manipulatethe orientation of the mounting plane of the crystals. The crystals arepreferably coupled in a way that allows them to function as a singlecomposite resonator. This allows for the construction of an accelerationand vibration resistant crystal oscillator.

One embodiment of an oscillator comprises an electronic circuitconfigured to initiate and sustain periodic oscillations, a plurality ofcrystal resonators having acceleration sensitivity vectors aligned in aconsistent and predictable relationship to the normal mounting plane ofthe resonator, wherein said crystal resonators are coupled to theelectronic circuit such that the acceleration sensitivity vectors of atleast two of the crystals are in an essentially anti-parallelrelationship, and wherein said crystal resonators function as a singlecomposite resonator controlling the frequency of oscillation. Theoscillator can have crystal resonators electrically coupled in parallelor the oscillator can have crystal resonators electrically coupled inseries. The crystal resonators can also be mechanically mounted byrotating at least one crystal resonator 180° around either the x or yaxis.

Also, the resonators can be mounted such that said resonators aredisposed on opposite sides of an oscillator substrate such that they maybe coupled to the oscillator circuit by their normal mounting means. Theresonators can be disposed side by side on the same surface of anoscillator substrate with the first resonator inverted 180° and mountedon its top with connections to the substrate. The oscillator can beconfigured such that the resonators are disposed back to back on thesame surface of an oscillator substrate said resonators being mounted ontheir sides so that electrical connection to all of the electrodes maybe made directly to the substrate.

An embodiment of the present invention also includes a method forimproving the acceleration resistance of a quartz crystal controlledoscillator comprising (a) manufacturing a plurality of crystalresonators having acceleration sensitivity vectors that point insubstantially the same direction relative to the mounting surface of theresonator, (b) determining the magnitude of the acceleration sensitivitycomponent in the axis normal to the mounting plane of the crystal forall crystals in the group, (c) selecting a first crystal and a secondcrystal that exhibit acceleration sensitivity magnitudes that aresubstantially the same within a predetermined tolerance, (d) mountingthe second said crystal such that its mounting plane is rotated 180°either around the x axis in the y-z plane or around the y axis in thex-z plane relative to the first crystal, aligning the two accelerationsensitivity vectors in a substantially anti-parallel arrangement, (e)coupling said crystal pair to the oscillator circuit so that thecombination performs as a composite resonator to control the frequencyof the oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a crystal oscillator assembly having a composite dualcrystal resonator.

FIG. 2 illustrates a cross-sectional view of the composite resonatorindicating the relative relationship of the crystal blanks and themounting structure.

FIG. 3 illustrates a cross-sectional side view of an embodiment of theinvention wherein the two matched crystals are mounted on opposite sidesof an oscillator substrate.

FIG. 4 illustrates an embodiment wherein the two matched crystals aremounted side by side on the same surface of the oscillator substrate.

FIG. 5 illustrates a further embodiment of the oscillator wherein thetwo crystals are mounted on the oscillator substrate with the packagesturned on their sides.

FIG. 6 illustrates an embodiment of an oscillator including a compositedual crystal resonator on a substrate containing the circuitry toimplement a Temperature Compensated Crystal Oscillator (TCXO).

FIG. 7 illustrates an embodiment of an oscillator wherein the compositecrystal resonator is included in an Oven Controlled Crystal Oscillator(OCXO).

FIG. 8 illustrates the axis definitions of a single rectangular quartzcrystal resonator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, an embodiment of a crystal oscillator 30 can beconfigured with a composite dual crystal resonator in which the crystalsare mounted with their normal mounting planes parallel to the surfaceplane of the circuit substrate 18. The oscillator 30 includes a firstcrystal resonator 10 and second crystal resonator 12 which are bothelectrically connected to the circuit with conducting jumpers 22 to formcomposite resonator 15. Preferably, the crystal resonators 10 and 12have been manufactured so that their Γ or acceleration sensitivityvectors 14 and 16 are pointing in the same direction relative to thenormal mounting planes of the resonator packages 36 and 38. Therefore,when the first crystal 10 is inverted with relation to the secondcrystal 12 and mounted on top of it, with the mounting planes parallelto each other, the acceleration sensitivity vectors 14 and 16 areessentially anti-parallel or pointing in opposing directions. While anarrangement with the acceleration sensitivity vectors exactlyanti-parallel is preferred, vectors that are nearly anti-parallel maystill provide the desired acceleration insensitivity. For instance, insome embodiments, the desired acceleration insensitivity benefits can beachieved where the acceleration vectors are within 5° off ofanti-parallel. However, where the acceleration vectors are arranged morethan 10° off of anti-parallel, the benefits can substantially diminish.Therefore, it is preferable that the acceleration vectors be within 10°of anti-parallel, and it more preferable that the vectors are within 5°of anti-parallel.

In one embodiment, the preferred crystal for the oscillator is arectangular resonator strip crystal known as an “AT” cut crystal. Thisparticular cut of crystal has a very low temperature coefficient withthe inflection temperature near +25° C. so that frequency variations areminimized in most applications. The temperature characteristic of aquartz crystal is primarily determined by the angle that the resonatorwafer is cut from a quartz bar relative to the crystal lattice. Whilethe implementation of an anti-parallel cancellation technique is wellsuited to the AT cut, it may also be accomplished with any other familyof cuts having an acceleration sensitivity in three axes.

The crystal resonators can be configured in an inverted position byrotating the first crystal 10 resonator 180° around either the x or yaxis and directly mounting the first crystal on top of the secondcrystal 12. FIG. 8 illustrates an embodiment of a rectangular resonator10 and a three axis coordinate system 50 which defines the axes relativeto the faces of the resonator. The z axis ({right arrow over (z)})points outward from the top of the package (a major face). The x axis({right arrow over (x)}) points outward from the side (the long minorface). The y axis ({right arrow over (y)}) points outward from the endof the package (the short minor face).

The illustrated configuration effectively causes cancellation of theacceleration sensitivity of the composite resonator due to the vectornature of the crystal acceleration parameter. Crystal electrode pads 27and 28 can be connected to the circuit with conducting jumpers 22 sothat they can be operated either in parallel or series configuration inthe oscillator circuit. The crystal oscillator 30 can also be configuredto include a circuit substrate 18 supporting passive and activeoscillator components 20.

FIG. 2 shows a cross-sectional side view of an embodiment of a compositeresonator 15 exposing the first internal quartz crystal blank 24 offirst crystal resonator 10 and the second internal quartz crystal blank26 of the second crystal resonator 12. Electrode pads 27 can beconnected to the electrode deposited on of crystal blanks 24 and 26.Circuit traces within the crystal packages connect pads 28 to theelectrode deposited on the other side of the crystal blanks 24 and 26.Preferably, the crystals are configured as part of a crystal resonatorpackage which can include circuit traces, electrodes, the crystals, andother resonator materials. In constructing the composite resonator,first crystal resonator 10 can be rotated or inverted 180° and placed ontop of the second crystal resonator 12. The first resonator 10 ispreferably rotated 180° around the x or y axis, as shown in FIG. 8, sothat planes 36 and 38 are mounted parallel to one another, and theacceleration sensitivity vectors 14 and 16 are aligned essentiallyanti-parallel. Although the vectors of the preferred embodiment arealigned exactly anti-parallel, other embodiments may have desiredacceleration benefits where the vectors are aligned within 5° and 10° ofanti-parallel. Conductive straps 22 can connect the electrode pads 27and 28 of the first crystal resonator 10 to the electrode pads 27 and 28of the second crystal resonator 12. The crystal resonators can also becoupled together using other means such as adhesive, use of thesubstrate, etc.

FIG. 3 illustrates an embodiment of an oscillator wherein the matchedcrystal resonators 10 and 12 are disposed on opposite sides of anoscillator circuit substrate 18. In this manner, the crystal resonators10 and 12 can each be attached to a substrate and circuit whilemaintaining the acceleration resistant positioning relative to oneanother. Preferably, the substrate has a uniform thickness so that themounting plane of the first crystal 36 is parallel to the mounting planeof the second crystal 38. As the orientation of the first crystal hasbeen inverted 180° around either the x or y axis, the accelerationsensitivity vectors 14 and 16 are essentially anti-parallel, pointing inopposite directions. Although the vectors of the preferred embodimentare aligned exactly anti-parallel, other embodiments may have thedesired acceleration benefits where the vectors are aligned withinroughly 5° and 10° of anti-parallel. The electrodes on crystal blanks 24and 26 can be connected through the crystal package to the electrodes 27and 28. The electrodes can then be connected together by conductivecircuit board via the circuit elements 40 and 42 which complete theconnection to the oscillator circuit either in a parallel or seriesconfiguration.

FIG. 4 shows a further embodiment of an oscillator wherein two matchedcrystal resonators 10 and 12 are mounted side by side on the samesurface of an oscillator substrate 18. Crystal resonator 10 is invertedor rotated 180° about the x or y axis and mounted on its top surface sothat the mounting planes 36 and 38 are parallel and the accelerationsensitivity vectors 14 and 16 are essentially anti-parallel. Althoughthe acceleration vectors of the preferred embodiment are aligned exactlyanti-parallel, other embodiments may have desired acceleration benefitswhere the vectors are aligned within roughly 5° and 10° ofanti-parallel. The crystal resonators 10 and 12 can be coupled directlyto the substrate and can also be coupled to one another. Preferably, thecrystal resonators have been manufactured so that the Γ vectors 14 and16 are pointing in a consistent direction relative to the mountingsurface of each crystal. The electrode pads 27 and 28 of the invertedcrystal 10 can be connected to the oscillator substrate 18 withconnecting jumpers 22.

FIG. 5 shows a further embodiment of an oscillator wherein the twomatched resonators 10 and 12 are mounted on their sides with theirnormal mounting surfaces 36 and 38 facing outward in oppositedirections. This allows crystal pads 27 and 28 to be electricallyconnected to the oscillator substrate 18 directly without requiringadditional connecting jumpers. Acceleration sensitivity vectors 14 and16 are thereby aligned anti-parallel in the horizontal plane. While itis most beneficial for the Γ vectors to be aligned exactlyanti-parallel, substantial acceleration benefits can be achieved if thevectors are aligned within 10° of anti-parallel. The crystal resonators10 and 12 can be mounted to the substrate and can also be coupled to oneanother. The oscillator can also include active or passive elements 20configured on the substrate 18.

FIG. 6 illustrates an embodiment of an oscillator 60 wherein the twomatched resonators 10 and 12 are mounted in an inverted position or backto back with the mounting plane of resonator 10 rotated 180° around thex axis in the y-z plane. The acceleration sensitivity vectors of thecrystal resonators 10 and 12 are arranged essentially anti-parallel.While it is most beneficial for the Γ vectors to be aligned exactlyanti-parallel, substantial acceleration benefits can be achieved if thevectors are aligned within 10° of anti-parallel. This compositeresonator is disposed on an interconnecting substrate 64. Conductivestraps 22 can connect the two resonators together and to theinterconnecting substrate 64. A TCXO circuit 62 can also be mounted onthe substrate 64 in order to produce a temperature compensated crystaloscillator 60. The TCXO circuitry 62 generates a correction signal tocompensate and minimize the frequency drift of the resonator as theambient temperature varies.

A TCXO with acceleration sensitivity vector cancellation based onembodiments of the invention has a g-sensitivity less than 0.05parts-per-billion (ppb) or 5×10⁻¹¹ per g of applied acceleration force.This is at least an order of magnitude improvement compared to otherTCXOs currently available. Also, when operating under random vibration,such a TCXO can improve the phase noise by more than 40 dB compared toconventional TCXOs. In an embodiment of an acceleration sensitivitycancelling TCXO, the frequency stability can be ±1 ppm over −40° to+70°. The input supply voltage can be +3.3 Vdc to +5 Vdc at 10 mA. Also,the low phase noise output can be CMOS compatible with 50/50±5% dutycycle. This can provide electronic frequency control for precise tuningor phase locking applications.

FIG. 7 illustrates a further embodiment of an oscillator 80 wherein thetwo resonators 10 and 12 are mounted in an inverted or back to backposition with the mounting plane of resonator 10 rotated 180° around thex axis in the y-z plane. The acceleration sensitivity vectors of thecrystal resonators 10 and 12 are arranged essentially anti-parallel.While it is most beneficial for the Γ vectors to be aligned exactlyanti-parallel, substantial acceleration benefits can be achieved if thevectors are aligned within 10° of anti-parallel. This compositeresonator is then mounted to a planar oven substrate 72. On thesubstrate is also a heat source 68, temperature sensor 66, oscillatorcircuit 70 and oven control circuit 68 which proportionally controls andstabilizes the heat source 68 to maintain the resonators at a precisetemperature even when the outside or ambient temperature varies. Theoven controlled crystal oscillator is housed within a package consistingof supporting header 74 and cover 76. Therefore, the oscillator 80 isless sensitive to acceleration forces and is also oven controlled.

It should be pointed out that while what has been described here areseveral embodiments of the invention, it may be possible to implementvarious modifications and variations without departing from the intentand scope of the invention. Although the invention presented herein hasbeen disclosed in the context of certain preferred embodiments andexamples, it will be understood by those skilled in the art that theinvention extends beyond the specifically disclosed embodiments to otheralternative embodiments and/or uses of the invention and obviousmodifications and equivalents thereof. Thus, it is intended that thescope of the invention herein disclosed should not be limited by theparticular embodiments described above, but should be determined only bya fair reading of the claims.

1. An oscillator comprising: an electronic circuit configured toinitiate and sustain periodic oscillations; a plurality of crystalresonators having acceleration sensitivity vectors aligned in aconsistent and predictable relationship to the normal mounting plane ofthe resonator; wherein said crystal resonators are coupled to theelectronic circuit such that the acceleration sensitivity vectors of atleast two of the crystals are in an essentially anti-parallelrelationship; and wherein said crystal resonators function as a singlecomposite resonator controlling the frequency of oscillation.
 2. Theoscillator in claim 1 wherein said crystal resonators are electricallycoupled in parallel.
 3. The oscillator in claim 1 wherein said crystalresonators are electrically coupled in series.
 4. The oscillator inclaim 1 wherein said crystal resonators are mechanically mounted byrotating at least one crystal resonator 180° around either the x or yaxis.
 5. The oscillator in claim 1 wherein said resonators are mountedsuch that said resonators are disposed on opposite sides of anoscillator substrate such that they may be coupled to the oscillatorcircuit by their normal mounting means.
 6. The oscillator in claim 1wherein said resonators are disposed side by side on the same surface ofan oscillator substrate with the first resonator inverted 180° andmounted on its top with connections to the substrate.
 7. The oscillatorin claim 1 wherein said resonators are disposed back to back on the samesurface of an oscillator substrate said resonators being mounted ontheir sides so that electrical connection to all of the electrodes maybe made directly to the substrate.
 8. A method for improving theacceleration resistance of a quartz crystal controlled oscillatorcomprising: manufacturing a plurality of crystal resonators havingacceleration sensitivity vectors that point in substantially the samedirection relative to the mounting surface of the resonator; determiningthe magnitude of the acceleration sensitivity component in the axisnormal to the mounting plane of the crystal for all crystals in thegroup; selecting a first crystal and a second crystal that exhibitacceleration sensitivity magnitudes that are substantially the samewithin a predetermined tolerance; mounting the second said crystal suchthat its mounting plane is rotated 180° either around the x axis in they-z plane or around the y axis in the x-z plane relative to the firstcrystal, aligning the two acceleration sensitivity vectors in asubstantially anti-parallel arrangement; and coupling said crystal pairto the oscillator circuit so that the combination performs as acomposite resonator to control the frequency of the oscillator.